Visible light communications can bring significant benefit to realize various intelligent lighting applications, such as easy commissioning, state information read out, and point and control etc., for light emitting diode (LED) lighting modules. One of the key challenges for designing a visible light modulation method is that the data modulation shall be compatible with the lighting functionalities, i.e. such that the data modulated into the light emitted are generally invisible for the human eye.
In particular, many advanced LED lighting systems allow lighting functionalities like dimming, e.g. for the sake of energy saving in an office environment or ambience creation in a restaurant.
There are three main dimming methods for LEDs, namely amplitude modulation dimming (AM-D), pulse width modulation dimming (PWM-D), and pulse density modulation dimming (PDM-D). Taking the dimming capability into account, one shall in practice typically design a data modulation method for a particular dimming method at a low cost and with minimum effect on the lamp efficiency. For instance, for compatibility with AM-D, one can modulate the amplitude of the light output around the desired light level for the purpose of data communication. As another instance, with respect to PWM-D, one can choose to modulate the widths or positions of consecutive pulses to embed data in the light signal. Different modulation methods are preferable for different dimming modes, since a low implementation cost and minimum loss in the lamp efficiency can be achieved with such consistency between the data modulation and lamp dimming methods.
An additional challenge, however, arises when a single lighting device is arranged to provide dimming with more than one dimming method, which may be applicable to increase the efficiency of the lighting device. A two mode dimming method, e.g. allowing both AM-D and PWM-D methods for dimming of the same lighting device for different situations is known. Typically, the selected dimming method may be set e.g. in response to which dimming level is requested.
Further, known visible light communication (VLC) schemes for LED lighting systems target at a fixed one or two mode dimming method and a corresponding fixed data modulation approach which are known to a receiver prior to the data reception process. In these fixed dimming mode and fixed data modulation approaches, the receiver always needs to acquire the dimming information, e.g. which dimming mode is used, of a lighting device prior to the data reception process, i.e. extraction of data modulated in the light, which is cumbersome, if not impossible.
From a data communication point of view, it would be desirable for a receiver to work independently with different dimming levels and dimming methods.